PEI / Ultem Injection Molding: A Buyer's Guide

PEI is generally specified when an application demands polycarbonate-like clarity at temperatures PC cannot survive, when aerospace components must meet strict flame-smoke-toxicity (FST) regulations without additives, or when medical devices require repeated autoclave sterilization. Sold most commonly under the brand name Ultem, it occupies a specific niche in the high-performance tier. This guide evaluates PEI as part of the material selection guide.

PEI Characteristics

PEI (polyetherimide) is an amorphous high-performance thermoplastic. It provides continuous-use temperature resistance alongside optical transparency (featuring a distinctive amber tint). It is inherently flame retardant with low smoke evolution and toxicity.

As an amorphous resin, PEI exhibits low and uniform shrinkage, facilitating tight tolerances—a distinct advantage over semi-crystalline high-performance peers like PPS and PEEK. Glass-filled grades are available to increase stiffness and strength.

Design and Application Constraints

• High cost. PEI sits in the high-performance price tier—well above engineering resins, though generally below PEEK.
• Notch sensitivity. Like polycarbonate, PEI is sensitive to stress concentrations; sharp internal corners must be radiused.
• Chemical limitations. While broadly resistant, it is attacked by certain chlorinated solvents and strong bases.
• Demanding processing. PEI requires very high melt and mold temperatures and stringent drying, necessitating specialized molding equipment.

Processing Realities

Molding PEI requires verification of supplier capability. It is processed at very high temperatures, meaning the supplier must utilize oil or electric mold heaters rather than standard water cooling.

Drying is strict. Wet PEI degrades structurally during injection. The molder must have rigorous dehumidifying dryer protocols. Because PEI shrinks amorphously, it is often easier to hold tight tolerances with it than with filled semi-crystalline alternatives.

One published datasheet is worth reading alongside the quote. SABIC’s sheet for unfilled Ultem 1000 gives a melt temperature of 350–410 °C and a mold temperature of 135–180 °C, with drying at 150 °C for four to six hours to reach 0.02% moisture. The number that matters most to a buyer is further down: molding shrinkage of 0.50–0.70%, quoted at the same value along the flow and across it. That symmetry is the amorphous advantage stated numerically — it is why a flat PEI part is easier to hold than a flat filled-nylon one, and it is the property you are paying the material premium for.

Ranges are grade-specific; confirm against the datasheet for the grade actually being quoted.

Comparing PEI

Against PEEK, PEI is more economical, transparent, and easier to hold dimensionally, while PEEK provides superior chemical resistance, wear, and ultimate temperature capability. Against PPS, PEI offers transparency and amorphous stability, whereas PPS delivers broader chemical resistance at a lower cost. Against polycarbonate, PEI functions similarly but at a significantly higher temperature class and price point.

Buyer FAQs

What is PEI (Ultem) used for in injection molding?

PEI is utilized in aerospace interiors requiring inherent FST compliance, reusable medical and dental devices undergoing repeated autoclave sterilization, high-temperature electrical insulators, and optical components in hot environments.

PEI vs PEEK, how do I choose?

Specify PEI when the part requires transparency, tight dimensional control, inherent FST behavior, or a lower cost within the high-performance tier. Specify PEEK when facing aggressive chemicals, severe wear, or the absolute highest continuous operating temperatures.

Is Ultem transparent?

Yes, unfilled PEI is transparent with a characteristic amber tint. It is not water-clear like acrylic or PC, but it is one of the few polymers offering optical transparency at such high temperatures. Glass-filled grades are opaque.

Is PEI hard to injection mold?

It requires specialized high-temperature equipment (hot tooling and high melt temperatures) and strict drying protocols. However, its amorphous shrinkage makes its dimensional behavior more predictable than filled semi-crystalline resins, provided the molder has the requisite equipment and experience.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, sell materials, or certify suppliers. Material behavior is grade-specific. Confirm properties and suitability against the manufacturer’s datasheet and your supplier’s engineering review.